A turbojet comprises, from upstream to downstream in the direction of the gas flow, a fan, one or more compressor stages, a combustion chamber, one or more turbine stages and a gas exhaust nozzle. The gases of the turbojet main flow, that is to say the flow of gases coming out of the turbine stages and originating from the combustion chamber, are exhausted via the gas exhaust channel terminating in a nozzle. The gases of the bypass flow are ejected either separately from the main flow or mixed with the main flow.
The exhaust channel usually comprises an external casing, that delimits the external envelope of the gas stream of the exhaust flow, and an internal casing, that delimits the internal envelope of the gas stream of the exhaust flow, to help with its discharge. The internal casing forms a central body of the exhaust channel and has the shape of a wall having a surface of revolution, that may be cylindrical, conical or more generally of aerodynamic shape suitable for the flow, with symmetry of revolution about the axis of the turbojet. Those skilled in the art also call this central body of the exhaust channel the “plug”. In what follows, it will be called the central body.
It is noted that the wall of the central body of the exhaust channel may also not have symmetry of revolution, for reasons of reducing the noise of the gas jet or of reducing the infrared signature; the central body may, in this case, have a corrugated, rectangular or elliptical cross section for example. The present invention applies particularly well to a central body with a wall with symmetry of revolution—which is usually the case in civil applications—but equally applies to a central body that does not have symmetry of revolution.
In certain turbojets, the central body fulfills another function that is to guide the turbojet vapor relief flow. Specifically, on certain turbojets, at the downstream end of its central shaft, a vapor relief orifice is provided through which various fluids such as oil vapor, certain cooling gases, etc. escape into the atmosphere. It is usually called an oil separator. In this case, either a pipe for guiding the vapor relief flow extends within the central body up to its end, for channeled guidance of the vapor relief flow, or no pipe is provided, the central body providing, via its internal surface, the guidance of the vapor relief flow. Vapor relief is usually carried out by aspiration, the pressure within the nozzle or of the central body being less than the pressure in the engine flow.
A constant problem that engine manufacturers have is noise reduction, particularly having in mind the comfort of the passengers and the inhabitants of zones overflown by aircraft. It is therefore necessary to attenuate the noise, particularly the noise in the nozzle, that consists of the noise generated in the combustion chamber, or combustion noise, that has low frequencies, and the noise generated in the high- and low-pressure turbines, or turbine noise, that has higher frequencies. With the known passive acoustic coverings, that is to say the devices whose geometry is fixed, the low-frequency combustion noise cannot be attenuated with the external casing of the exhaust channel because the volume in this location is not sufficient. It is therefore the higher-frequency turbine noise that can be treated on this casing, for example with the aid of a thin layer of a honeycomb-shaped material. The problem then posed is the attenuation of the low-frequency combustion noise.